It is known that phase change materials (PCMs) act as heat sinks at their phase-change temperature.
PCMs may be organic, such as paraffins and silicones, or inorganic, such as hydrated salts and metal alloys.
The expression “thermo-cyclic process” refers to any cyclic process during which certain steps are exothermic, i.e. accompanied by heat evolution, while certain other steps are endothermic, i.e. accompanied by heat consumption.
Typical examples of thermo-cyclic processes for which the invention may be carried out include:                gas separation processes such as PSA (Pressure Swing Adsorption), VSA (Vacuum Swing Adsorption), VPSA (Vacuum Pressure Swing Adsorption) and MPSA (Mixed Pressure Swing Adsorption); and        any process employing a chemical conversion coupled to pressure swing adsorption cycles, such as those mentioned above, for shifting the equilibrium of chemical reactions.        
Pressure swing adsorption separation processes are based on the phenomenon of physical adsorption and are used to separate or purify gases by pressure-cycling the gas to be treated through one or more adsorbent beds, such as zeolite, active carbon, activated alumina, silica gel or molecular sieve beds, or the like.
In the context of the present invention, unless otherwise stipulated the term “PSA process” denotes any gas separation process by pressure swing adsorption, employing a cyclic variation of the pressure between a high pressure, called the adsorption pressure, and a low pressure, called the regeneration pressure. Consequently, the generic term “PSA process” will be used also to denote the following cyclic processes:                VSA processes in which the adsorption is carried out substantially at atmospheric pressure, called the “high pressure”, i.e. between 1 bara and 1.6 bara (bara=bar absolute), preferably between 1.1 and 1.5 bara, and the desorption pressure, called the “low pressure”, is below atmospheric pressure, typically between 30 and 800 mbara, preferably between 100 and 600 mbara;        VPSA or MPSA processes in which the adsorption is carried out at a high pressure substantially above atmospheric pressure, generally between 1.6 and 8 bara, preferably between 2 and 6 bara, and the low pressure is below atmospheric pressure, typically between 30 and 800 mbara, preferably between 100 and 600 mbara;        PSA processes in which the adsorption is carried out at a high pressure substantially above atmospheric pressure, typically between 1.6 and 50 bara, preferably between 2 and 35 bara, and the low pressure is above or substantially equal to atmospheric pressure, and therefore between 1 and 9 bara, preferably between 1.2 and 2.5 bara; and        RPSA (rapid PSA) processes, which denote PSA processes with a very rapid cycle, generally shorter than one minute.        
In general, a PSA process makes it possible to separate one or more gas molecules from a gas mixture containing them, by exploiting the difference in affinity of a given adsorbent or, where appropriate, several adsorbents, for these various gas molecules.
The affinity of an adsorbent for a gas molecule depends on the structure and composition of the adsorbent, and also on the properties of the molecule, especially its size, its electronic structure and its multipolar moments.
An adsorbent may for example be a zeolite, an active carbon, an activated alumina, a silica gel, a carbon or non-carbon molecular sieve, an organometallic structure, one or more oxides or hydroxides of alkali or alkaline-earth metals, or a porous structure containing a substance capable of reversibly reacting with one or more gas molecules, such as amines, physical solvents, metal complexing agents, and metal oxides or hydroxides for example.
The thermal effects that result from the enthalpy of adsorption or the enthalpy of reaction generally result in the propagation, at each cycle, of a heatwave at adsorption that limits the adsorption and of a cold wave at desorption that limits the desorption.
This local cyclic phenomenon of temperature swings has a not insignificant impact on the separation performance and the specific separation energy, as document EP-A-1 188 470 implicates.
One particular case covered in the context of the present patent is the storage of gas in and removal of gas from a reactor or adsorber at least partly containing one or more adsorbents. Here too, a thermo-cyclic process involves an adsorbent material with heat release during gas storage (increase in pressure) and heat absorption during gas removal (decrease in pressure).
In both these cases, one solution for reducing the amplitude of the thermal swings consists in adding a phase change material (PCM) to the adsorbent bed, as described by document U.S. Pat. No. 4,971,605. In this way, the adsorption and desorption heat, or some of this heat, is adsorbed in latent heat form by the PCM at the temperature, or in the temperature range, of the phase change of the PCM. It is then possible to operate the PSA unit in a mode closer to isothermal.
The PCMs may be microencapsulated and thus available in powder form. More precisely, microencapsulated PCMs take the form of microbeads of a polymer forming an impermeable shell containing wax or a linear saturated hydrocarbon with between 14 and 24 carbon atoms. Said microencapsulation is generally obtained by phase inversion of an emulsion using processes known to those skilled in the art. The mean size of a microbead is around 5 microns. The general shape of the capsule is spherical, but it could instead be ellipsoidal or even potato-shaped. The diameter of said capsule may then be defined as that of the sphere containing it.
When the temperature increases, the hydrocarbon contained in the bead absorbs the heat and stores it. When the temperature decreases, the hydrocarbon contained in the microbead releases the stored latent heat by changing from a liquid phase to a solid phase. During the phase change period, the temperature remains approximately constant (depending on the composition of the wax) and allows the temperature to be regulated to levels well defined by the nature of the hydrocarbon (or hydrocarbons when there is a mixture thereof) and in particular by the length of the chain and the number of carbon atoms. One commercial example of a PCM corresponding to this description is the product Micronal® from BASF.
However, microencapsulated PCMs cannot be introduced as such into an adsorbent bed as it would be difficult to control the distribution thereof. Furthermore, they would be entrained by the gas streams flowing through the adsorber. It is therefore necessary beforehand to produce “agglomerates”. The term “agglomerate” is understood hereafter to mean a solid with a size of greater than 0.1 mm that may adopt various forms, in particular a bead, extrudate, pellet or milled form, obtained by milling and screening blocks of larger sizes, or a plate form obtained by cutting precompacted sheets, or the like. In the context of the invention, the particles involved are more particularly of spherical appearance, which will be denoted by the term “beads”.
A first solution involves making an intimate mixture of the adsorbent—in powder or crystal form—and of the PCMs and agglomerating the mixture. The products obtained by dry compression prove generally to be too fragile for industrial use. Agglomeration in a liquid or wet phase poses the problem of how to activate the active phase of the agglomerate. Indeed, it is known that most adsorbents have to be heated to a high temperature before use in industrial processes for achieving the required performance. The required temperature level is generally above 200° C., and often around 300 to 450° C. These temperature levels are not compatible with the mechanical integrity of the PCMs.
A second solution consists in making only PCM agglomerates, in the form of a structure that can be easily handled and introduced into an adsorber.
However, the processes for manufacturing agglomerates according to the current state of the art (pelletizing under pressure, extrusion, etc.) do not result in agglomerates with mechanical and/or thermal properties sufficient to be used effectively in thermo-cyclic processes.
One of the reasons for this is that the operating conditions for manufacturing these agglomerates—by the processes conventionally used to manufacture rods, beads or pellets of adsorbents or catalysts—are limited by the intrinsic strength of the PCMs themselves. By dint of their nature, they would be unable to withstand the pressures or temperatures needed to form strong agglomerates.
Another reason stems from the particular nature of the shell (for example polymeric) and from the deformability of the capsules, which makes processes such as pressure agglomeration not very effective.
More precisely, the agglomerates formed by conventional means while respecting the pressure and temperature constraints inherent in PCMs are too friable for industrial applications, in particular those of the PSA type. A fraction of the agglomerates break up, thereby causing problems of poor distribution of the process fluid in the adsorber or problems of the filter being blocked by creating fine dust consisting of PCMs.
A third approach consists in integrating the PCM microparticles in a preexisting solid structure such as a cellular structure, namely a “honeycomb” structure or a foam, a lattice, a mesh, etc., for example by bonding to the walls. Such materials that can be produced in the laboratory cannot be used in large industrial units (with a volume greater than 1 m3 and more generally greater than 10 m3) for manufacturing or cost reasons.
The aim of the present invention is therefore to alleviate these drawbacks by producing agglomerates made up of PCMs of shape, diameter and density such that a mixture of adsorbent particles and these agglomerates remains homogeneous in space and over time during operation of PSA-type units. In particular, the mechanical strength of such agglomerates (crush and attrition resistance) enables them to remain intact under the operating conditions. This invention allows such agglomerates to be produced in large quantities and at low cost.